The present invention relates to a two-shaft type rotary machine for use in a vacuum pump system, and more particularly to a machine called a "ROOTS" type machine which includes two rotors constituting in combination one stage and having respective shafts rotating in opposite directions to each other.
In order to allow a rotary machine to perform a stable operation, it is most important from the viewpoint of design to give sufficient rigidity to the rotating shaft. However, any excessive increase in the diameter d of the rotating shaft with respect to the outer diameter D of the rotor leads to a reduction in the theoretical displacement per revolution. It is, therefore, necessary to select an appropriate shaft diameter d with both the displacement and mechanical strength taken into consideration. Envelope, involute and cycloid profiles are generally known as rotor profiles of ROOTS type pumps. In the case of envelope and cycloid profiles, the ratio D/d of the rotor outer diameter D (the diameter of the tip circle) to the rotating shaft diameter d, that is, the shortest diameter (the diameter of the root circle), is primarily determined, whereas, in the case of an involute profile, the ratio D/d can be varied as desired by changing the pressure angle (.alpha.) of the involute curve defined hereafter within a certain range.
Referring to FIG. 7, which shows a typical conventional involute type rotor, each of the tip portions 12a and 13a is defined by the circle of the rotor's outer diameter Do (the diameter of the tip circle) which intersects the involute curve portions 12c (13c), while each of the root portions 12b and 13b is defined by two circular arcs (radius r.sub.o) which intersect the involute curve portions 12c (13c) and which also contact the circle of the diameter do. Pressure angle (.alpha.) is defined as an angle formed between a line f tangential to base circles Rb of rotors 12 and 13 and a center line g perpendicular to a line h passing through both centers of the rotors 12 and 13. The base circle Rb is defined as a circle passing the meeting points of the involute curves 12c (13c) and circles 12b (13b) and concentric with the rotor 12 (13). The theoretical displacement volume per revolution is equivalent to 6 times (in the case of a three-lobe rotor) the trapping space 14 defined between the housing 11 and the rotor 12 and is generally expressed as follows: EQU V=KD.sup.2 L
V: theoretical displacement volume per revolution PA1 D: outer diameter of rotor PA1 L: rotor thickness (depth of the space occupied by the rotor) PA1 K: coefficient of theoretical displacement. PA1 wherein n is the number of lobes of the rotor: n.gtoreq.3.
The theoretical displacement coefficient K is determined by the rotor profile. Maximization of the theoretical displacement coefficient K enables an increase in the displacement of the pump.
In the case of the above-described involute type pump having the configuration exemplarily shown in FIG. 7, however, a sealed space 15 is defined at the area of meshing engagement between the rotors 12 and 13 and this space 15 is compressed by the meshing of the rotors 12 and 13 during the trapping process and then released toward the suction side. This phenomenon causes various drawbacks such as generation of vibration and noise, an increase in the power consumption and a reduction in the displacement and thus leads to losses in the pump operation. In particular, the prior art suffers from the problem that the sealed space 15 increases as the pressure angle (.alpha.) becomes smaller.